Across the UV Spectrum

By Austin Richards, Ph.D., Oculus Photonics Across the UV Spectrum Digital imaging from near-ultraviolet to deep-ultraviolet Figure 1 shows the relative spectral response for the Sony XCD-SX910UV camera, which is a two megapixel machine vision camera with enhanced UV response. • Digital imaging here are many interesting and diverse systems now are commercially available applications for ultraviolet imaging, a that allow one to take UV digital stills • UV technology that is now over a century and video with a live preview mode that old. These include imaging tiny makes it simple to dynamically adjust shot • UV fluorescence T composition, exposure and focus. These scratches and imperfections in surfaces, the visualization of sun-induced skin damage advances have been a great boon to forensics • Cameras and faded bruises, and the detection of and medical workers who have been trained • Spectral response trace evidence and residue on surfaces. I in frustrating and slow methods of reflect- am referring specifically to reflected UV ed-UV film photography. Consider that imaging, where a UV illumination source methods using SLR film cameras equipped is used in conjunction with a UV-sensitive with black glass UV pass filters will require camera to produce a pure UV image. This bracketing the exposures, high f/numbers technique is different from UV-fluorescence which increase depth of field to compensate imaging, where a UV excitation source is for focus shifts, and severe challenges in used to stimulate visible-light emissions correctly framing a shot of a moving target, from a fluorescent surface, which can then since the eye cannot see an image through be visualized either by human eyes or color the barrier filter. There also is a mythology cameras equipped with barrier filters that that one MUST use very expensive fused block the UV excitation. silica lenses for UV imaging1. For these Recent developments in digital ultraviolet reasons, many technical people decided imaging technology have made reflected- years ago that reflected-UV imaging is UV imaging much more affordable and too difficult and clumsy and have stopped accessible to a diverse user group. Various trying to use it, which is one of the reasons 1 Fused silica lenses are often incorrectly referred to as being the same as fused quartz lenses. They are chemically identical, but fused silica is superior in purity and optical homogeneity. 10 — ADVANCED IMAGING www.advancedimagingpro.com JUNE 2008 Build the RIGHTSYSTEMtechspec Every time techspec ® In Figure 2, you can see that the spectral purity of low-pressure Hg lamps is quite high. Note that the 254nm line is almost 100 times more intense than the next most intense line at 436nm. ts that most photographic filter companies a two megapixel machine vision camera have discontinued manufacturing black with enhanced UV response. As is the glass UV pass filters. case with the SX910UV, silicon CCDs are TELECENTRIC EO USB 2.0 CAMERA There are many ways to divide the generally more responsive to visible and LENSES ultraviolet portion of the UV spectrum near-infrared light than they are to UV. In into sub-bands, and every field of science order to image under lighting conditions has its own nomenclature, which I will that include visible or near-IR light (such not attempt to catalog here. The two main as in sunlight), the imaging system must wavebands considered in this article are have a filter or filter stack that blocks this the near-UV band (generally considered to undesired light so that the desired UV band be 300-400nm) and the deep UV, which is will dominate the images’ spectral content. roughly 200-300nm. 300nm is a convenient In my experience, conventional color COMPACT FIXED LED LIGHT ENGINE dividing point, since both the atmosphere video or SLR lenses seem to work fairly FOCAL LENGTH LENS and standard optical glass both cut off well for near-UV imaging (330-400nm), but rather sharply around this wavelength. fail to transmit well below about 320nm, NEW LENS SELECTION GUIDE A reflected-UV imaging system consists which is where the BK-7 lens glass material of the following components: becomes highly absorbing. If the bandpass Match Imaging • A detector that is sensitive to UV light in is narrow, as is the case with near-UV LED Needs with Lenses the desired waveband illumination, the chromatic aberration of and Cameras • A lens that transmits the desired color lenses used in the UV is quite tolerable. www.edmundoptics.com/selectionguide waveband of UV light and focuses it Imaging in the deep-UV band below the correctly on the detector glass cutoff wavelength requires expensive NEW! • A filter that passes the desired UV optics made of fused silica or calcium 404 - page waveband while simultaneously blocking fluoride. These lenses are commercially Master Source out-of-band light to the required extent available but the selection of focal lengths Book for the lighting conditions is quite limited, and I know of no commer- • Sufficient UV illumination in the desired cially available deep-UV zoom lenses. Many Request your waveband of these lenses are not achromatic, requiring FREE copy today! For digital UV imaging systems, the most refocusing if the bandpass is varied. common detector is a silicon CCD array, The solar spectrum at sea level has 800.363.1992 though there are other technologies (such significant levels of near-UV light, particu- www.edmundoptics.com/AI as gallium nitride) that show promise. The larly above 330nm, making it possible to CCD may be back-thinned to increase its take UV images in direct sunlight down to response to UV light below 300nm. Figure around 330nm. A UV bandpass filter with 1 shows the relative spectral response for good out-of-band blocking is required to the Sony XCD-SX910UV camera, which is maintain the purity of the image. Below Visit us at The Vision Show Booth 207 JUNE 2008 www.advancedimagingpro.com ADVANCED IMAGING — 11 Figure 3: These images show the expected absorption behavior: 330nm, it is necessary to use artificial il- the BK7 glass becomes progressively more absorbing as the lumination, as the solar spectrum is steeply wavelength is decreased, with a very steep drop off below 300nm. falling to begin with and absorption by the (Courtesy David Hayes) ozone layer becomes increasingly strong. To put it another way, it is very dark outside in same at 254nm as it is at 436nm, the resulting the deep-UV band below about 300nm, even images have a spectral content that is almost at high noon! completely deep-UV at 254nm. The medium UV imaging indoors almost always requires pressure Hg lamps that peak at 306nm have artificial sources, since conventional home similar spectra but with pressure-broadened and office lighting and exterior windows are lines. Both of these lamps can be dangerous specifically designed to maintain low levels and protective eyewear should be worn when of UV. Standard “blacklight” fluorescent working with them to avoid “welder’s eye.” bulbs generate near-UV light using a I have found that a UV bandpass filter mercury vapor discharge, predominately is not required to create very pure 254nm at the 365nm line. These bulbs are coated images with the Sony SX910UV camera, inside with a dark purple filter that greatly provided that the only light source present is reduces visible light emissions. Inexpensive the mercury lamp and that the target is not germicidal mercury-vapor discharge lamps fluorescent. Figure 3 shows a series of three are readily available that generate either images taken at 350nm, 306nm and 254nm 254nm or 306nm radiation depending on the of 2-inch circular windows made of BK7 glass vapor pressure. The spectral purity of low- and fused silica. The background material is pressure Hg lamps is quite high, as shown in sheet aluminum that has been lightly sanded Figure 2. Note that the 254nm line is almost to make the surface a diffuse reflector. 100 times more intense than the next most These images show the expected absorption intense line at 436nm. Since the SX910UV behavior: the BK7 glass on the left become pro- camera’s spectral response is roughly the gressively more absorbing as the wavelength is decreased, with a very steep drop off below 300nm. Test targets like this serve as an excellent test of the spectral purity of a UV imaging system: camera, lens, optics and bandpass filters (if any). The camera used was the SX910UV. The 350nm image was taken through a Hoya CLEMEX intelligent microscopy U-350 filter with a blacklight bulb for illumination. The 306nm and 254nm images were taken with medium-pressure and low-pressure mercury lamps and special Barr Associates UV bandpass filters, re- Quantitative spectively. Note that the BK7 is just as transparent at 350nm as the Image Analysis fused silica – it is certainly not necessary to use fused silica lenses for near-UV imaging, especially above 330nm. in biomedical research Figure 4 shows an example of one of the effects one can see with near-UV and deep UV imaging. The substrate material is a marble tile, which simulates a marble statue with traces of whole-egg- based paint on it. Archaeologists have used shortwave reflected- UV imaging to look at ancient Greek statues to determine how they were painted. The dots are from left to right, egg yolk, egg white and scrambled 1 egg. The second row of dots is the same as the first row, but with a much thinner layer applied. Inorganic crystalline materials tend 1. Human skin tissue to reflect UV down to quite short wavelengths, which is why the 2. Brain tissue 2 tile looks fairly “white” in the three wavebands.

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